Substrate effect on the optical response of thin palladium films exposed to hydrogen gas

The optical response of hydrogenated thin palladium (Pd) films deposited on two different substrates, Si and Al2O3 is studied by means of reflectance measurements. The effect of the substrate is investigated with the samples exposed to hydrogen gas in ranges of pressure corresponding to the various phases of PdHx. The reflectance of both samples decreases upon hydrogen exposure and the relative reflectance drop reaches 17% for the Pd/Al2O3 sample exposed to 3.3 × 104 Pa of hydrogen gas. The response time to hydrogen loading is extracted from the measured data for various pressure increments and follows a non-linear variation vs. pressure. Interestingly, this relation can be related to structural changes that occur during hydrogenation and in particular during the α to β phase change. The response time of the Pd/substrate system to hydrogen exposure is found to be highly dependent on the type of substrate, with values up to 6 times greater in the case of Al2O3 compared to Si substrate. X-ray reflectivity measurements suggest that the substrate-induced surface roughness of the films is responsible for the observed behavior.